Bypassing Brake Pedal Monitoring System: A Potential Source of Error?

Hi zusammen

Occasionally, there is mention of a performance issue, with the cause being identified after a long investigation as a mis-synchronization of the switching points between the brake light and brake pedal switches.

When examining the wiring diagram (for the Golf 3), I noticed that it would actually be possible to short-circuit the two inputs of the engine computer together and connect them to a single switch.

When the brake is not engaged,
- the brake light switch is open, and the brake light circuit pulls the engine computer input 44 against ground
- The brake pedal switch is closed, and also pulls the Engine Control Unit (ECU) input 20 towards ground.

When the brake is applied
- there is a 12 Volt supply at the brake lights or at the engine computer input 44.
- when the brake pedal switch opens, and an internal pull-up also pulls the engine computer input 20 "high" – conveniently also to 12 volts instead of 5 volts, as one might expect in a digital system.

Therefore, the voltage at both brake input terminals of the engine computer (when both switches are functioning normally) is always 0 or 12 volts simultaneously.

My idea now is to disconnect pin 44 of the motor computer from the brake light switch and connect it to pin 20 of the pedal switch. "Inputs that are not accustomed to signals from the permanent power supply will not be affected by voltage even when the ignition is switched off (the brake lights are powered by permanent power)."

Does anyone here know reasons that would argue against this simplification, or why the brake pedal monitoring was implemented so complexly with 2 switches?

Hi Ulf.
I believe that this version of the circuit was chosen to minimize power consumption when the 'Motor Off' function is active.
What could happen if the two connectors are connected together as suggested?

I think the only thing that happens as a limitation is that the brake lights will only light up when the ignition is 'on'.
Exactly there, I see a problem.
Assume your engine stalls (or shuts down, whatever) and you're standing up or at an intersection. Then the cars behind you won't notice if you turn off the ignition, because you're standing on the brake...

I also don't know what the TÜV says about this, whether it might be a change to the emissions standard.

What if you switched a relay that is activated by 'ignition on'?
send a signal to the engine from the brake lights.
This ensures that no power is consumed when the engine is running, and the lights also function...


However, it would be necessary to check whether the Motorcompi can actually provide the 12V, especially since it doesn't actually have a source for it there...


...would be quite interesting too...
Bye.
Micha

Hi Micha,

in my plan, keep the brake lights on with a constant power supply.

I would only disconnect the connection from the brake lightswitch to the engine computer (pin 44) and connect it with the pedalswitch (pin 20).

This wouldn't affect the TÜV at all.

Okay then...
take it on
I don't think there are any reasons against it then...

if you do, let me know what happened...
PS: Have you ever had any issues with the 'VW double' (referring to a specific VW model)?

[Translating...]denkt bitte daran, dass die steuergeräte von ABS und automatikgetriebe auf die korrekte funktion des bremslichtschalters angewiesen sind. ganz besonders das ABS !

CU Gremlin

[Translating...]

Gremlin wrote:

denkt bitte daran, dass die steuergeräte von ABS und automatikgetriebe auf die korrekte funktion des bremslichtschalters angewiesen sind. ganz besonders das ABS !

Hi

danke für den Tip.
Ich fahr Schaltgetriebe.
Mein ABS-Stromlaufplan enthält nur eine Anzapfung des Bremslichtschalters, aber keine Verbindung zum Pedalschalter der EDC.
Wenn ich also der EDC das (korrekte) Signal des Pedalschalters zuführe und das ABS an der Bremslichtader lasse, dürfte doch nix passieren, oder?

@micha: ich hate noch keinen Streß, aber ich baue auch gern vor, um die Möglichkeit auszuschließen

Mein ABS-Stromlaufplan enthält nur eine Anzapfung des Bremslichtschalters, aber keine Verbindung zum Pedalschalter der EDC.
Wenn ich also der EDC das (korrekte) Signal des Pedalschalters zuführe und das ABS an der Bremslichtader lasse, dürfte doch nix passieren, oder?


das sollte dann funktionieren.

hauptsache, ihr verändert nicht die funktionen der schaltung. diese plausibilitätsprüfung hat schon ihren sinn. da wir (wie auch fast alle neueren benziner) drive-by-wire haben gehört diese überwachung mit zur sicherheitsrelevanz.
wie ihr gemerkt habt kann man nicht gegen die bremse gas geben.
weiterhin wird bei einem ausfall des gaspedal-potis ein erhöhter leerlauf eingesteuert (ebenso beim ausfall des drosselklappenstellers im benziner, dann durch federkraft). das bremspedal übersteuert aber in jedem fall die gemischaufbereitung. dadurch kann man auch bei ausfall des gaspedals noch in die werkstatt humpeln. beim druck auf die bremse wird sofort der motor (notfalls gewaltsam) abgedrosselt.

CU Gremlin

as you have noticed, you cannot 'brake' the gas.
Furthermore, in the event of a failure of the accelerator pedal pot, an increased idle setting will be implemented (as well as in the case of a failure of the throttle valve in a gasoline engine, then through spring force). However, the brake pedal will always override the mixture preparation. Therefore, it is also possible to drive to the workshop even if the accelerator pedal fails. When the brake pedal is pressed, the engine (if necessary, forcibly) is immediately shut down.

See you, Gremlin
That would match my problem, which I posted here /viewtopic.php?t=919 . However, during a braking maneuver, I experienced a sudden increase in speed, which gave me a huge rush of adrenaline.
Does the brake pedal issue also occur when I use the clutch? I've been experiencing a phenomenon lately where the engine RPM drops, and then for about 1/2 to 1 second, there's no power output, until normal power is restored.

Hi everyone,

Regarding the second point raised by Micha_Passat, I can already confirm that it is not relevant for TÜV approval. (even though Ulf has already circumvented this)

The brake lights must generally only function with the ignition on. Whether they also function without the ignition is irrelevant. I used to drive Fords (shame on me...), and they generally did not have brake lights without the ignition.

Greetings
Private